The invention relates to the field of chemical hydrides, and more particularly to a method and device to regenerate chemical hydrides using molten salt-based electrochemical cells.
Hydrogen is a product of the reaction of metals and chemical compounds in water. The common chemistry lab experiment of floating a piece of Na on water to produce hydrogen gas demonstrates this principle, and the Na transforms to NaOH in this reaction. The reaction is not directly reversible, but NaOH can, for example, be removed and reduced in a solar furnace back to metallic Na. In this reaction, two Na atoms react with two H2O molecules to produce H2. The hydrogen molecule produces a H2O molecule in combustion, which can be recycled to produce more H2 gas. However, the second H2O molecule necessary for the oxidation of the two Na atoms must be added. Therefore, Na has a gravimetric hydrogen density of about 2.5 mass %. The same process carried out with Li leads to a gravimetric hydrogen density of about 4 mass %. Thus, while metal hydride storage technology is an option as a chemical storage means for hydrogen, the lack of efficient techniques for regenerating the chemical hydrides remains an impediment to developing this storage technology.
Simple chemical hydride systems such as the hydrolysis (the reaction with water) of LiH, NaH, or MgH2, are currently recycled (from the LiOH, NaOH, or Mg(OH)2 products) using carbo-thermic or hydro-thermic reduction. These recycling processes operate at high temperatures (1000° C.) and generate the pure metals (i.e., Li, Na, or Mg). For example, estimates of the energy input necessary to generate the metal Li by the conventional method for recycling are as follows. Chemical generation of Li metal from LiOH begins with dehydrogenation to Li2O according to the reaction LiOH→½Li2O+½H2O(g). The standard enthalpy for this reaction is 65 kJ/mol-LiOH. Two possible chemical reduction methods to generate Li metal from Li2O are Li2O+½C→2Li+½CO2 and Li2O+H2→2Li+H2O(g). The standard enthalpy for the first reaction is 402 kJ/mol-Li2O and for the second is 357 kJ/mol-Li2O. The low energy efficiency of these reactions is accordingly clear. In a second step, the pure metals are hydrogenated to the corresponding hydrides. Formation of the pure metals from the spent hydride material is an extremely energy intensive process, considerably more energy intensive than directly regenerating the hydrides. Thus, the energy-efficiency of the overall processes is low.
Complex chemical hydride systems, such as the hydrolysis of LiBH4 or NaBH4, or the hydride/hydroxide reaction between LiBH4 and LiOH.H2O, cannot be recycled easily by carbo-thermic or hydro-thermic reduction. To recycle NaBH4 (from the hydrolysis product NaBO2), reaction with MgH2 has been developed. This reaction is exothermic (which wastes energy unless the exothermic heat is captured and utilized) and produces MgO. The MgO byproduct must subsequently be recycled back into MgH2 in an energy intensive process. Again, the overall energy-efficiency is low.
Currently, a large obstacle to the widespread use of chemical hydrides for hydrogen generation is the low recycling efficiency. In any widespread application the spent (i.e., dehydrogenated) hydride must be recycled. Because chemical hydrides release hydrogen exothermically, recycling requires energy input (i.e., the hydrogenation reaction is endothermic). One way to input the required energy is to couple the endothermic hydrogenation reaction with at least one exothermic reaction. However, the methods thus far developed to do this operate at high temperatures, require multiple steps, and therefore, have poor overall efficiency. Another option is providing the energy electrochemically.
An electrochemical process has recently been described to recycle NaOH, the product of NaH hydrolysis. Although the overall energy-efficiency has not been evaluated, the process currently still first produces Na metal, and a second step is required to form NaH.
There accordingly remains a need for improved methodologies and devices to regenerate chemical hydrides, whether simple hydride systems or complex hydride systems.